Demonstration of quantum-limited discrimination of multi-copy pure versus mixed states

Abstract

We demonstrate an optical receiver that achieves the quantum Chernoff bound for discriminating coherent states from thermal states in the multi-copy scenario. In contrast, we find that repeated use of the receiver approaching the Helstrom bound for single-copy measurement is sub-optimal in this multi-copy case. Furthermore, for a large class of multi-copy discrimination tasks between a pure and a mixed state, we prove that any Helstrom-bound achieving single-copy receiver is suboptimal by a factor of at least two in error-probability exponent compared to the multi-copy quantum Chernoff bound. This behavior has a classical analog in the performance gap between soft-decision and hard-decision receivers for detecting a multi-copy signal embedded in white Gaussian noise.

11 pages, 4 figures

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• Discriminating mixed qubit states with collective measurements
• First-photon target detection: Beating Nair's pure-loss performance limit
• Quantum-enhanced quickest change detection of transmission loss
• High Fidelity Artificial Quantum Thermal State Generation using Encoded Coherent States